1. Field of the Invention
The present invention relates to a self priming centrifugal pump comprising a metal plate case with a substantially cylindrical shape defining internally thereof a pressure chamber with an inlet port and an outlet port, within this pressure chamber a rotatable impeller and a radial diffuser being arranged at an axial end thereof, an axial ejector device being located at the opposite axial end thereof, and an intermediate torus-shaped interspace being arranged in an intermediate position to direct the outlet flow issued by the radial diffuser backwards to the pressure chamber.
2. Description of the Prior Art
In the known pumps as above, the flow emitted by the diffuser and accelerated by the impeller has an outlet speed with a high rotational component. Thus, most of the air trapped in the pumped fluid is directed to the pressure chamber and passes through the ejector several times before being discharged through the outlet port, thereby lengthening the priming times of the pump. U.S. Pat. No. 2,941,474 discloses a centrifugal pump having a torus-shaped interspace which forms a vortex chamber with an annular central opening extending around the periphery of the ejector. The trapped air present in the fluid subjected to a swirling action and is finally discharged through the outlet port in the pressure chamber without being previously separated. This arrangement is inadequate and does not afford significant reduction of the priming times.
The pumping apparatus described in the British Patent No. 1.201.721 and in the Italian Patent Application No. 85605 A/89 comprises an interspace which is provided with a plurality of substantially radial blades adapted to counteract the rotational component of the fluid flow thereby promoting separation of air. Similarly the reconvey interspace applied to the pump described in the Italian Patent No. 1.225.597 is provided with a number of substantially radial blades which form recirculator channels, however this pump makes use of an axial diffuser which is arranged to face the impeller on the low pressure side thereof, and therefore the flow entering the recirculator channels has a speed direction with a relatively high rotational component. Moreover, the structure of these latter pumps is excessively complicated and brings about remarkable fluidodynamic losses.
The self priming centrifugal pumps known from the German Patent No. 3718273 and the European Patent No. 0323384 have an interspace with a single blade that offers the advantage of a lower complexity and reduced frictional losses. However, such interspace has a wall with one or more outlets for the fluid with relatively limited sizes which are located only in the upper half of the interspace wall. Thus, the trapped air tends to swirl and to be retained into the interspace thus elongating the priming times and reducing the efficiency of the pump.
One aim of the present invention is to eliminate the above mentioned drawbacks by providing a self priming centrifugal pump incorporating an eject, or which is capable of operating with high efficiency arid is very simple in structure.